As it is well known to those skilled in the art, measuring of the flow of a fluid through temperature can be performed by exploiting two physical principles: the dissipative thermodynamic principle and the calorimetric principle.
The dissipative thermodynamic principle is based in particular on the difference between the fluid temperature and that of a body immersed therein, being heated with a constant, or anyway known, quantity of energy. This difference only depends on the heat exchange conditions between the body and the fluid, and it is thus functionally linked to the fluid speed as well as to its density.
The calorimetric principle is based instead on the temperature difference setting up in a fluid flow between the upstream and downstream of a heating region against the application of a known quantity of energy. The applicability of this principle requires the thermal uniformity of the fluid being measured.
Prior art already provides some solutions applying the dissipative thermodynamic principle to the measuring of a fluid motion.
There are in fact some devices essentially based on two resistive sensors, one of which is heated and the other is used to sense the fluid temperature, such as for example those described in the Application Note No. AN9801 by Farruggia et al.
However, these known devices have some drawbacks in terms of cost and measurement reliability. Moreover, known devices are often affected by measuring offsets and they require heavy and delicate calibration interventions. Another drawback is the difficulty in miniaturizing them, thereby they have a considerable size and dimensions.
These acknowledged drawbacks actually limit the possibility to widen the field of application of sensors being manufactured by exploiting the thermodynamic principle according to the prior art.
Prior art also provides some solutions applying the calorimetric principle to the measuring of a fluid motion.
For example, Patent GB1116178 describes a device for measuring the flow of a fluid, comprising three tubular sections made with two metals having different thermoelectric properties and arranged in series and alternated with each other. In this way, two junctions of a thermocouple have been formed, which are positioned at a predetermined distance from each other with an intermediate section associated with a heater and susceptible to be heated.
Although advantageous under several aspects, the device being provided by prior art is not without drawbacks due both to the complexity of manufacturing and coupling of the different parts and to the considerable total mass, which limits its application owing to the intrinsic thermal inertia.
Moreover, in operation, a considerable power is to be supplied to the heating element.
It should also be said that, when gaseous fluids are to be measured, the device sensitivity is considerably reduced because of the thermal masses involved to such an extent that it has proved to be inadequate for properly measuring low fluid speeds.
To obviate to this drawback, a sensor device exploiting the dissipative thermodynamic principle, comprising a thermocouple as a temperature sensor and heating means of a junction of said thermocouple, has been suggested in the European Patent Application no. EP1705463 in the name of Lanzani et al.
In particular, the sensor device being described in this application comprises the heating element on a support, some conductive tracks of metallic material being formed on the surface thereof in order to form thermocouple conductive tracks. The heating element can be made with an electrical resistor positioned on the support near the thermocouple.
Although advantageous under several aspects, also this known solution is not without drawbacks. In particular, the resistive-type heating element is an additional cost since it is manufactured separately and it is difficult to position and assemble, with considerable limits in terms of accuracy, giving rise to uncertainties and positioning and/or thermal contact tolerances which actually negatively influence the accuracy of the measuring being performed.
Moreover, the heating element, which is anyway added to the sensor device support, increases the mass and thus the thermal inertia near the hot junction, giving rise to a further disadvantage in terms of sensor response time. Finally, the sensor device final size should take into account the size of the heater as a separate element with related pads to be placed on the support of the sensor device itself.
Also known from the US Patent application published under No. US 2005/0109103 is a liquid level sensor using a plurality of thermocouple junctions. It should be remarked that, according to this document, heating elements are realized on the surface of the device, thus protruding from the same so increasing the global size of the device itself. Hence, the miniaturizing of this solution is limited and its cost quite high, the positioning of such heating elements also requiring ad hoc design and thus repetitivity problems.